The present invention relates to bicycle gear cranks, and more particularly to a bicycle gear crank which includes a crank arm with a boss to mount to a crank shaft, and at least one gear supported by the crank arm.
Bicycles have a gear crank as a front gear, a free wheel as a rear gear and an endless chain which is wound around these gear crank gear and the free wheel gear, as the bicycle driving mechanism. The gear crank gear is fixedly supported by the crank arm. The crank arm has a base end provided with a boss for mounting to a crank shaft which is rotatably supported by a bottom bracket in a frame. The crank arm also has a tip end provided with a pedal. When the gear crank is rotated in normal direction by pedaling, the rotation is transmitted by the endless chain to the free wheel as normal-directional rotation, the rear wheel is turned in the normal direction and thus the bicycle moves in the forward direction. Recently, most bicycles are equipped with some speed changing mechanism. When the mechanism is provided by an external type, i.e. a derailleur system, the free wheel is usually equipped with multiple gears, and in some cases the gear crank is also equipped with multiple gears. This provides a multiple-speed system with as many shift positions as given by a multiplication between the number of gears in the gear crank and the number of gears in the free wheel.
The crank arm may include, integrally therewith, a gear mount which extends radially from the boss (see Patent Document 1). In this case, the gear mount may have some threaded holes while the gear may have some though-holes, and bolts may be inserted through these holes to fixedly support the gear to the gear mount on the crank arm. In other arrangements, a gear mount and a crank arm may be separate members and are connected with each other incapably of relative rotation to each other.
As is well known, the gear may be provided by an oval gear which has its teeth formed on its oval circumference. In bicycles, it is considered that the rider on a bicycle can exert a maximum pedaling force when the pedal on its circular path is ahead of the crank arm boss while the crank arm makes one complete rotation for example. With this idea, an oval gear can provide an advantage: If the oval gear is positioned in such a manner that its major axis comes to a vertical position when the rider can exert a maximum force onto the pedal, then the gear turns as fast as a true-circle gear formed with a greater number of teeth, within a range where the maximum force is being applied. In other words, it is believed that an oval gear will provide a faster speed as compared to a true-circle gear which has the same number of teeth. Due to such a characteristic, an oval gear is often used in racing bicycles.
However, the position of the crank arm when a maximum force is exerted to the pedal varies from one bicycle rider to another, and for this reason, there has been a requirement for adjustment capability on circumferential positioning between the crank arm and the gear.